Allogeneic hematopoietic stem cell transplantation (aHSCT) is a potentially curative therapy used to treat many malignant and non-malignant diseases. Despite advances in HSCT, such as the use of post-transplant cyclophosphamide, graft-versus-host disease (GVHD) remains a significant cause of morbidity and mortality in patients receiving aHSCT. In fact, as many as half of the ~8,000 aHSCTs performed in the U.S. each year will result in GVHD. Pre-transplant conditioning, including irradiation and chemotherapy for hematological tumors, causes widespread death of dividing cells and release of endogenous danger signals, as well as bacterial translocation due to gastrointestinal epithelial barrier dysfunction, which promotes production of a pro- inflammatory cytokine storm. These cytokines drive the differentiation and expansion of allo-reactive donor T cells leading to damage of particular host tissues characteristic of GVHD. We propose that Stimulator of Interferon Genes (STING), an innate immune sensor, contributes to the inflammatory response following conditioning and transplant. STING signaling has been implicated in several diseases as a consequence of its role in cytoplasmic DNA sensing from both intracellular and other sources, including from the phagocytosis of dying cells. STING has been well studied in the context of viral infection and cancer; however, its role in GVHD remains unknown. The aims for this proposal are: (1) to determine how pre-HSCT conditioning and transplant regulates STING activation in the colon and other tissues, which downstream signaling pathways are important in promoting GVHD and (2) to determine if STING deficiency or its modulation in vivo can attenuate GVHD without abrogating vaccination induced anti-tumor, i.e. ?graft-versus-leukemia (GVL)?, effects. Based on our exciting and reproducible preliminary findings indicating STING activation in recipient tissues promotes GVHD, primary objectives of this proposal are to identify how STING is activated after aHSCT and which pathway(s) are involved in its regulation of GVHD. The studies in this proposal will provide new information regarding the involvement of STING in the context of aHSCT which may be useful in other conditions of widespread cell death, such as during chemotherapy, traumatic injury, and autoimmune disease. Specifically, the findings from these experiments will contribute to the understanding of how innate immune sensors and non-hematopoietic cells in GVHD initiating tissues can regulate adaptive T cell responses. The long term objective of my studies will be to develop a new therapy that can be translated into the clinic to prevent or reduce GVHD, thereby increasing the overall number of available stem cell donors to augment the use of aHSCT.